For purposes of illustration of the technical context of the disclosure herein, FIG. 1 shows in a view from above an example of an aircraft 10, the fuselage 11 of which comprises an outer coating R.
Habitually, the outer coatings used in aircraft, for example a coating R of the type used for the fuselage 11 of the aircraft 10 represented in FIG. 1, undergo degradations in service. These can be grouped into two main categories, i.e. degradations caused by optical deterioration and degradations caused by mechanical deterioration.
In the case of optical deterioration, the degradations correspond to faults associated with a change such as the brightness or color of the coating. For example, this can involve yellowing and/or a loss of brightness of the coating, inter alia.
In the case of mechanical deterioration, the degradations correspond to physical defects of the coating. For example, this can involve cracking and/or detachment, inter alia.
The origin of these degradations of the coating can be explained by the combination of several stresses, in particular associated with photo-oxidation, temperature and humidity, and the mechanical aspect. The photo-oxidation corresponds to the chemical aging of the coating when it is subjected to solar radiation. The stresses caused by temperature and humidity are derived from a substantial and rapid variation of temperature and humidity. Finally, the mechanical stresses correspond to the stresses induced by the structure of the aircraft, to which the coating is subjected.
In order to study the degradations of the coating of an aircraft during its service life, tests are conventionally carried out, in particular in order to characterize the durability of the coating. However, the tests which are performed habitually are often lengthy, and take into account only a single parameter, typically the temperature, humidity, or also ultraviolet radiation. However, an approach of this type is not representative of the behavior in service of the outer coating, and does not make it possible to track down the degradations observed on the coating.
In addition, the prior art does not teach a solution for carrying out accelerated aging tests of a coating taking into account the combination or succession of stresses which give rise to the degradations (photo-oxidation, variation of temperature and humidity, mechanical stresses, etc.) to which it is subjected, for the purpose of reproducing these degradations.
In reality, at present, there are firstly enclosures which make it possible to carry out photo-oxidation on a coating sample for an aircraft in order to test its durability in accelerated aging conditions.
FIG. 2 illustrates in cross section an example of an enclosure 12 for accelerated photo-aging according to the prior art, in order to carry out photo-oxidation on a coating sample R of an aircraft.
In this enclosure 12, test pieces 13 for sampling of the coating R are secured on a test piece support 14, and subjected to exposure to a xenon lamp 15 situated inside the support 14, as represented in FIG. 2. The temperature of the enclosure 12 is for example approximately 55° C., in order to permit exposure of the test pieces 13 to the photo-oxidation, and carry out the tests of durability of the coating R after accelerated aging.
In addition, there are also enclosures which make it possible to apply mechanical stresses by thermal shocks on a coating sample for an aircraft, in order to test its durability in accelerated aging conditions.
FIG. 3 illustrates in cross section an example of an enclosure 12 for accelerated aging by thermal shocks according to the prior art, in order to apply mechanical stresses on a coating sample R of an aircraft.
In this type of enclosure 12, two sub-enclosures 12a and 12b are provided, placed on top of one another and separated by a wall 16 which is provided with an opening 16a. In addition, a support 14 for sampling test pieces of the coating R is provided in the enclosure 12. This support 14 can be displaced vertically according to the double arrow F between the sub-enclosures 12a and 12b, in order to subject sampling test pieces to thermal shocks. The displacement of the support 14 then makes it possible to close the opening 16a in the wall 16 by one of its flanks, as in the case of its positioning represented in broken lines in FIG. 3 within the sub-enclosure 12b. 
In order to be able to test pieces which are situated in the support 14 to thermal shocks, the temperature T1 of the sub-enclosure 12a is clearly distinct from the temperature T2 of the sub-enclosure 12b. In particular, the temperature T1 can be selected as approximately 70° C., whereas the temperature T2 can be selected as approximately −55° C. This considerable difference between the temperatures T1 and T2 makes it possible to obtain a thermal shock during the passage of the support 14 from one sub-enclosure to the other.
Thus, for example it is possible to create a thermal shock which gives rise to mechanical stresses on the sampling test pieces of the coating R, by placing the support 14 in the sub-enclosure 12a with the temperature T1 of approximately 70° C., with the lower flank of the support 14 closing the opening 16a in the wall 16. Then, the support 14 is displaced vertically towards the sub-enclosure 12b by any type of mechanism, for example such as, inter alia, by an articulated arm or by a jack which thrusts the support 14 upwards or downwards, which sub-enclosure is at a temperature T2 of approximately −55° C., the opening 16a in the wall 16 then being closed by the upper flank of the support 14, and the sampling test pieces of the coating R being subjected to thermal shock of the rapid passage from T1 to T2.
However, these two types of enclosures previously described are not entirely satisfactory for carrying out accelerated aging tests of outer coatings of an aircraft which are sufficiently representative of the real wear sustained in service by these coatings. In particular, the prior art does not teach a solution which makes it possible to combine at least these two types of stresses (photo-oxidation and mechanical stresses by thermal shocks) even though this combination of stresses is representative of the real aging observed in service.